High pressure ethylene polymerization using catalysts of the ziegler type

Abstract

Ethylene is polymerized, in the presence of hydrogen, at a temperature of 180° to 340° C and a pressure of 200 2,500 bars with a Ziegler catalyst in a stirred reactor having at least one first reaction zone and at least one second reaction zone. The reaction mixture is fed to a separator at a pressure of 80 to 400 bars. The improvement results from partially liquifying at least a part of the gas coming from the separator at a temperature of -50° to +20° C, releasing the pressure to a value of 10 to 60 bars, separating the partially liquified gas into a gas phase and a liquid phase, and recycling the gas phase to the second reaction zone and the liquid phase to the first reaction zone. The resulting polymer has a low melt index, a high polydispersity index, and superior physical properties compared to polymers prepared by conventional processes.

We claim: 
     
       1. In a process for the polymerisation of a reaction mixture comprising ethylene, in the presence of hydrogen, at a temperature of 180° to 340° C under a pressure of 1,000 to 2,500 bars in the presence of a catalyst in at least one stirred reactor comprising at least one first reaction zone and at least one second reaction zone, which process comprises releasing the pressure and passing the reaction mixture to a first separator under a pressure of 80 to 400 bars to form a gas phase and a liquid phase, passing the liquid phase coming from the first separator to a first funnel under a pressure of 1 to 15 bars to form a polymer and a gas phase, recovering the polymer coming from the first funnel, and recompressing the gas phase coming from the first funnel and passing it to the first reaction zone, the improvement comprising passing the gas phase coming from the first separator to a second funnel via a first condenser to form a liquid phase and a gas phase, recovering the liquid phase from the second funnel, partially liquifying at least a part of the gas phase coming from the second funnel at a temperature of -50 to +20° C by means of a second condenser, releasing the pressure to a value of 10 to 60 bars and separating the flow from the second condenser in a second separator to form a liquid and a gas phase, recompressing the gas phase coming from the second separator and passing the recompressed gas phase from the second separator to the second reaction zone, mixing the liquid phase coming from the second separator with the gas phase coming from the first funnel, recompressing said mixture of liquid phase and gas phase, and passing said recompressed mixture to the first reaction zone. 
     
     
       2. Process according to claim 1, further comprising injecting hydrogen into the gas phase coming from the second separator before it is recompressed. 
     
     
       3. Process according to claim 1, wherein said ethylene is polymerised in the presence of 1 to 20% by weight of an inert hydrocarbon. 
     
     
       4. Process according to claim 1, wherein said ethylene is copolymerised with a member selected from the group consisting of an α-olefine and a non-conjugated diolefine. 
     
     
       5. Process according to claim 1, further comprising injecting ethylene between the reactor and the first separator, at a pressure below that of the first separator. 
     
     
       6. Process according to claim 5, wherein the injected ethylene is obtained from the gas phase coming from the second separator. 
     
     
       7. Process according to claim 1, further comprising, at the end of the reaction, injecting into the reaction mixture at least one member selected from the group consisting of alkali metal salts and alkaline earth metal salts of a carboxylic acid, said member being selected in order that its reaction products with the catalyst components remain in the polymer. 
     
     
       8. Process according to claim 7, wherein said injected member is injected in the pure state. 
     
     
       9. Process according to claim 7, wherein said injected member is diluted with a hydrocarbon prior to being injected. 
     
     
       10. Process according to claim 1, wherein the recompressed mixture passed to the first reaction zone constitutes from 25 to 200% by weight of the gas phase passed to the second reaction zone. 
     
     
       11. A process according to claim 1, wherein said catalyst is a catalyst system of the Ziegler type comprising a halogen compound of a transition metal and an organo-aluminum activator. 
     
     
       12. A process according to claim 1, wherein said catalyst is a peroxide or perester initiator.